1. Field of the Invention
This invention relates to the field of ceramics and more particularly to the field of advanced ceramic composites for high temperature, pressure, and/or severe environment applications.
2. Description of the prior art
There has been an increasing interest in structural materials that can withstand severe conditions of temperature, pressure, and environment and can substitute for high-temperature alloys that require imported critical materials. The nonoxide ceramics (nitrides, carbides, borides, etc.) have high-technology applications because of their superior mechanical properties, hardness, and toughness.
According to the processes of manufacturing ceramic compounds hitherto known, the powders employed for the manufacture of ceramic bodies are dressed or prepared, then compressed by application of pressure or by use of a binder and finally sintered by a heat treatment. The compressed bodies are heated in furnaces, generally operated with electricity, until the required sintering temperature is reached; that is, the compressed bodies are subjected to external heat.
Advanced ceramic materials such as silicon carbide, silicon nitride, and SiAlON (silicon-alumina-oxynitride) have been considered for applications as materials which can replace metallic alloys requiring additions of chromium or other strategic metals to achieve required strengths.
Titanium based alloys have been proposed for use in certain elevated temperature applications where dimensional stability is important since they have moderately low density and good oxidation resistance and certain alloys have good properties at elevated temperatures. To increase the performance of jet aircraft, there is an ever present need to improve the strength and creep resistance of high temperature titanium alloys. Therefore, metallurgists have added increasing amounts of alloy elements (particularly aluminum) to titanium in order to increase the strength. However, when the aluminum content exceeds 6%, an ordered precipitate of Ti.sub.3 Al is formed.
In U.S. Pat. No. 2,872,726 Goliber discusses hard refractory compositions produced by powder metallurgy from pulverulent mixtures of alumina and lower oxides or suboxides of titanium and from pulverulent mixtures of the aforesaid two ingredients and chromium. It is known that the lower oxides or suboxides of titanium, for example, those approximating titanium monoxide in composition, are hard substances. However, these materials by themselves lack strength and are much too brittle for practical use.
In U.S. Pat. No. 2,982,014, Meyer-Hartwig found that with mixed or metallic ceramics, the required increase in temperature can be obtained by employing powders reacting exothermically with each other. The use of exothermically reacting powders is particularly advantageous for the sintering process proper as the diffusion during the reaction results in a strong bonding between the metallic and non-metallic particles.
In other prior art, such as the U.S. Pat. No. 2,973,570 to Nachtman, the presence of substantial percentages of metal oxides, mixed metal oxides, inorganic compounds of metal oxides and other inorganic non-metallics have always been considered to be detrimental to the useful properties of the base metal or alloys containing such substances. It was found that structural materials possessing unusual and desirable properties may be produced by powder metallurgical processes from compositions of aluminum and aluminum alloy powders containing in finely divided form, oxides of those metals having high melting points and good solid solubility in aluminum where such oxides contain no non-metal other than oxygen.